A video signal may represent a first series of images converted to electrical energy that also may be transmitted, compressed, decompressed and received by a device which then may use the video signal to generate a second series of images which is ideally substantially similar to the first series of images. During the process of generating the second series of images from the first series of images, (i.e., any combination of the transmitting, compressing, decompressing, receiving, and displaying) a noise component may be added to the video signal so that the signal includes an image component representing the series of images and a noise component. Depending on the magnitude of the noise component, the second series of images may contain noise components which are visible to the viewer of the second series of images.
As a consequence, video noise reduction systems have been developed to attempt to reduce the noise component of the video signal. In U.S. Pat. No. 4,987,481 to Spear et al., the invention is a noise reduction system which averages from 2 to 9 frames, or series of images, of the video signal to determine the noise content in the series of images. This system, however, only works on the portions of the series of images that are similar in color and brightness in each image. In U.S. Pat. No. 5,025,312 to Faroudja, the invention is a noise reduction system which uses recirculation for the portions of the series of images containing low level picture details and selectively cores the moving portions of the series of images. Coring consists of passing only those portions of a signal that have an absolute amplitude level which is greater than some threshold magnitude and zeroing components having magnitudes less than the threshold. Although this system may reduce the noise component of the series of images, the coring or recirculation process may add noticeable artifacts or spurious spatial frequency components to the series of images. Both coring and recirculation may also undesirably reduce the apparent resolution of the series of images.
In U.S. Pat. No. 4,523,230 to Carlson et al., the invention is a noise reduction system which uses an octave spectrum analyzer to separate the video signal into subspectra signals. The system cores one or more of the subspectra signals and then generates a noise reduced video signal by recombining the subspectra signals. This system also may add noticeable artifacts or spurious spatial frequency components to the video signal caused by the coring process.
In U.S. Pat. No. 4,163,258 to Ebihara et al., the invention is a noise reduction system which uses an orthogonal transform to transform the video signal into a number of transformed signal components. Then a process similar to coring is performed on some of the transformed signal components and the transformed signal components are transformed back to a video signal. This system may also add noticeable artifacts or spurious spatial frequency components to the video signal caused by the coring process on the transformed signal components.
These prior art noise reduction systems also reduce the amplitudes of some of the data samples which make up the image component while reducing the noise component of the video signal. The reduction of the amplitudes of some data samples which made up the image component in those systems distorts the series of images represented by the video signal, i.e., reduces the detail or AC energy of the video signal. The reduction, thus, in effect, adds a different type of noise component to the video signal, i.e., a noise component which changes the spatial frequency spectrum of the video signal.